On-Detector Guide Windows (ODGW)

Introduction

Each of the GSAOI detectors contains programmable ODGWs. The ODGWs provide tip-tilt information for up to four natural guidestars to GeMS. The ODWGs can be read out at up to 800 Hz during a science exposure. Sizes of either 4x4, 6x6, 8x8, 12x12, 16x16, 32x32, or 64x64 pixels can be specified, although most science observations will be completed using 6x6 or 8x8. The ODGWs are used to measure star centroids for either Canopus tip-tilt correction or for fast guiding to correct differential flexure between GeMS and GSAOI.The figure below shows a typical ODGW dark (left) and flat-field (right). As one can see in the figure, only immediately adjacent pixels in the science image are affected.

On-Detector Guide Window with the main array exposed for a dark frame (left) and a flat-field frame(right).

Each of the four ODGWs (one per detector) can be read out with different exposure times, as long as each exposure time is an integer multiple of the shortest exposure time. In practice, the four ODGWs are read out synchronously using non-destructive reads. These data are processed in a way to synthesize exposure times that are an integer multiple of the basic read time.

The ODGWs are read out using Fowler Sampling. The minimum frame time, T, to read an NxN pixel ODGW with M-M Fowler non-destructive reads (NDRs) is given by the following relation:

T = (2M + 1)[5.0N2 + 37.06 N + 17.37] μs

here, 2M + 1 is number of passes through the whole array; 1 reset pass, M Fowler NDRs at the start of the integration, and M Fowler NDRs at the end of the integration. Each pass through the array takes 5.0N2 + 37.06N + 17.37 μs, which consists of 5.0 μs/pixel to read the NxN ODGW pixels, a row overhead of 37.06 μs/row, and a frame of overhead of 17.37 μs. A set of simulations were performed to determined the exposure times needed to operate the ODGWs at fixed frame rates (likely to be used in practice). The derived parameters have been used to simulate the ODGW centroiding accuracy for stars of different brightnesses and for different filters. These values will be used as starting point for the ODGW characterizations during GSAOI + GeMS commissioning.

Reads of the ODGWs are interleaved with reads of the science image during resets and Fowler sampling. This requires a reduction of the ODGWs frame rate to ~200 Hz during these periods when the science image is accessed. Otherwise, the science image read out time would be unacceptably long.

On-Detector Guide Window Sensitivities

The On-Detector Guide Window performance has been estimated using the image performance model (see GSAOI Sensitivity section) to generate star frames that have then been centroided and the RMS centroiding accuracy determined from 200 simulated guide star frames. Limiting magnitudes that achieve a centroiding accuracy of ~ 2 mas (i.e., 0.1 pix) in integration times of 0.01 sec. (i.e., when used as a GeMS NGS) and 30 s (i.e., when monitoring flexure variations) are listed in table below. The table also list the magnitude of the detector saturation for both cases. Note that the values listed in the table are a good approximation of what it is expected, but will be confirmed during GSAOI commissioning.

On-Detector Guide Window Sensitivities (2 miliarcsec RMS)

Filter

Lim. magnitudes (mag)

10 ms integration

Sat. magnitudes (mag)

10 ms integration

Lim. magnitudes (mag)

30 sec integration

Sat. magnitudes (mag)

30 sec integration

Z

15.1

8.0

22.7

16.7

J

14.2

7.1

21.2

15.8

H

14.5

7.3

21.0

16.0

K(prime)

13.7

6.6

20.4

15.2

K(short)

13.6

6.5

20.3

15.2

K

13.5

6.5

20.2

15.1

J-continuum

12.0

4.6

20.0

13.5

H-continuum

12.0

4.7

19.7

13.4

CH4(short)

13.5

6.3

20.5

15.0

CH4(long)

13.1

5.9

20.1

14.6

K(short) continuum

11.2

4.1

19.1

12.7

K(long) continuum

11.0

3.8

18.8

12.5

He I 1.083 μm

12.3

5.3

20.6

13.8

H I Pγ

11.9

4.6

20.2

13.2

H I Pβ

11.8

4.6

19.4

13.3

[Fe II] 1.644 μm

11.8

4.5

19.4

13.2

H2O

12.3

5.1

19.8

13.8

He I (2p2s)

11.2

3.9

18.9

12.7

H2 1-0 S(1)

11.3

4.1

19.0

12.8

H I Brγ

11.3

4.0

18.9

12.7

H2 2-1 S(1)

11.0

3.8

18.9

12.6

CO Δv=2

11.6

4.4

18.8

13.2

On-Detector Guide Window Availability

The availability of suitably faint guide star in the near-infrared was investigated by Spagna, A. (2001, STScI_NGST-R-0013B, “Guide Star Requirements for NGST: Deep NIR Star Counts and Guide Star Catalogs”) for the NGST (now JWST). Spagna tabulated cumulative star counts from which probabilities can be calculated that the 2' diameter GeMS field for any science object will contain at least one guide star brighter than a particular limit. The results of this investigation are presented graphically in the figure below. The figure shows that the probability of finding at least one K ≤ 13 mag guide star, suitable for use as a GeMS NGS with the broad-band filters, in the GeMS field is > 11% while the probability of finding at least one K ≤ 20 mag guide star, suitable for flexure monitoring with the broad-band filters, in the GeMS field is high (> 97%).